The only time I've ever had a problem with the HBO GO app was with the Season 3 Premiere of Game of Thrones, which I believe was put on HBO Go at the exact same time as it was airing live on tv. At the time, we only had SD receivers, so our only way of watching it in HD was on the app.

But, it wouldn't even load. I think Microsoft had to make a statement on twitter or something about how Game of Thrones had broken the app with all the people watching it on the app at the same exact time or something. I think I wound up waiting about two hours or something before I could stream it without issue.

That was the only time I've ever really had any issues with the app being slow, or not loading.